The present invention relates to a discharge lamp and a lamp unit. In particular, a discharge lamp and a lamp unit used as a light source for an image projection apparatus such as a liquid crystal projector and a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector and a DMD projector has been widely used as a system for realizing large-scale screen images, and a high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. In the image projection apparatus, light is required to be concentrated on a very small area of a liquid crystal panel or the like, so that in addition to high intensity, it is also necessary to achieve nearly a point light source. Therefore, among high-pressure discharge lamps, a short arc type ultra high pressure mercury lamp that is nearly a point light and has a high intensity has been noted widely as a promising light source.
Referring to FIGS. 8A to 8C, a conventional short arc type ultra high pressure mercury lamp 1000 will be described.
FIG. 8A is a schematic top view of a lamp 1000. FIG. 8B is a schematic side view of a lamp 1000. FIG. 8C is a cross-sectional view taken along line c-c′ of FIG. 8A.
The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions (seal portions) 120 and 120′ made of also quartz glass and connected to the luminous bulb 110. A discharge space 115 is inside the luminous bulb 110. A mercury 118 in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm3 as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space 115.
A pair of tungsten electrodes (W electrode) 112 and 112′ are opposed with a certain gap in the discharge space 115, and a coil 114 is wound around the end of the W electrode 112 (or 112′). An electrode axis 116 of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion 117 where the electrode axis 116 and the Mo foil 124 are welded.
The sealing portion 120 includes a glass portion 122 extended from the luminous bulb 110 and the Mo foil 124. The cross-sectional shape of the sealing portion 120 is circular, as shown in FIG. 8C. In the sealing portion 120, the glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. The principle of the reason why the luminous bulb 110 can be sealed by the sealing portion 120 will be briefly described below.
Since the thermal expansion coefficient of the quartz glass constituting the glass portion 122 is different from that of the molybdenum constituting the Mo foil 124, the glass portion 122 and the Mo foil 124 are not integrated. However, by plastically deforming the Mo foil 124, the gap between the Mo foil 124 and the glass portion 122 can be filled. Thus, the Mo foil 124 and the glass portion 122 are attached to each other, and the luminous bulb 110 can be sealed with the sealing portion 120. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing. Since the glass portion 122 and the electrode axis 116 of the W electrode 112 are not attached tightly to each other, a gap (not shown) is generated between the glass portion 122 and the electrode axis 116 by a difference in the thermal expansion coefficient.
The Mo foil 124 attached to the glass portion 122 of the sealing portion 120 has a rectangular planar shape, and is positioned in the center of the sealing portions 120 and 120′, as shown in FIG. 8C. The Mo foil 124 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion 117 is positioned. The Mo foil 124 and the external lead 130 are welded to each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The external lead 130 is electrically connected to a member (not shown) positioned in the periphery of the lamp 1000.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112′ via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112′. As the pressure of the mercury vapor of the lamp 1000 is higher, the emission efficiency is higher, so that the higher pressure of the mercury vapor is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
As a result of in-depth research, the inventors of the present invention found that the lifetime of the conventional lamp 1000 is shortened by the fact that the sealing structure of the sealing portions 120 is destroyed.
More specifically, the cross-sectional shape of the sealing portions 120 of the lamp 1000 is circular, so that the length of the sealing portion 120 in the thickness direction is constant (in other words, the thickness of the glass portion 122 of the sealing portion 120 is constant). In addition, since the sealing portion 120 is sealed by the attachment between the Mo foil 124 and the glass portion 122, as shown in FIGS. 9A and 9B, an internal stress 40 (from the glass portion 122) occurs uniformly on the Mo foil 124 in the direction perpendicular to the surface of the foil (the Z direction in FIGS. 9A and 9B). For this reason, as shown in FIG. 9C, when expansion and contraction of the Mo foil 124 are repeated with use of the lamp 1000, the gap 119 between the glass portion 122 on the luminous bulb 110 side and the electrode axis 116 proceeds in the direction shown by an arrow 119a (i.e., the longitudinal direction of the Mo foil 124) between the glass portion 122 and the Mo foil 124 that are simply attached. When the gap 119 proceeds and reaches the welded portion 132 between the Mo foil 124 and the external lead 130, the entire Mo foil 124 is oxidized. Thus, the conductivity of the Mo foils 124 is lost, so that the lamp 1000 stops its operation.
To deal with compactness of the lamp size corresponding to compactness of image projection apparatuses, reducing the size of the sealing portion 120 is in demand. To meet this demand, when the size of the sealing portion 120 is reduced, as shown in FIG. 9B, the thickness T of the glass between the side face 124a of the Mo foil 124 and the surface 122a of the glass portion 122 becomes small. Therefore, a crack 45 proceeding from the side face 124a of the Mo foil 124 reaches the surface 122a of the glass portion 122, so that the sealing structure of the sealing portion 120 can be destroyed.